GPS

[Sbwood007]

Do any of you use a GPS system? If so which one? Do they work? Advantages, Disadvantages.

 

Thanks,

Stephen

[sweatinoutliquor]

Yup they work great (I use a garmin RINO 120)

 

Advantages:

build in 2 way radio

works great as a safety net: I mark waypoints before trips using software, during trips at locations such as where I parked, gear stashes, etc. I really only use it as a backup though. Sometimes they aren't very reliable, and you really need to know how it works to know when it's playing games with you.

 

Disadvantages: It's heavy enough that I don't bring it lot's of times, just because I don't use it. I thought the radio thing was going to be priceless, but honestly, I just don't really use that function. If I had to do it again, I would get one of those garmin geko things. They weigh nothing, fit in your pocket, and have basically the same GPS functionality as my garmin. Anyway, search this forum or the gear critic forum to find pages of discussion on this stuff

 

[gslater]

SOL -

 

In what way has your Rino been unreliable? Are you saying that GPS is unreliable, or that particular unit?

[jonmf76]

I use the Garmin III+, but I have never used it for mountaineering. I have always wondered if you could navigate through total whiteout on a mountain.

 

This model has downloadable maps. One of these maps showed a river going up and over a mountain ridge, so I am not sure how much I'd trust them. That was in Alaska however so those maps often aren't as refined.

 

Has anyone used one to navigate in total whiteout?

 

I know reception gets to be an issue when the atmosphere is dense from crummy weather. Can usually only get connected to a few satellites instead of 8 or more when it is sunny out.

[Ovr40]

I use an eTrex Summit. It's reasonably simple to use. I have used it solely as an altimeter (occasionally) and as a backup for map and compass routefinding.

I typically mark key waypoints and take the time to label them with names. It is good "find your way back" insurance without much cost of weight or bulk.

I once walked out at night, in a whiteout, using these waypoints. It was just a practice experience (I walked into harsh weather intending to emergency bivy for a few hours and return in the dark), but the GPS worked very well. It returned me to within 10 feet of the marked waypoints. It was a strange experience trusting the GPS, because in those conditions my "instincts" told me the GPS was wrong. Several times I was positive it had failed me until I was right on top of the waypoint.

Fun times.

 

[sweatinoutliquor]

Garmins are weird... For example, if you are trying to navigate to a waypoint and you lose satelite signal, it won't tell you that you have lost your signal until 30 seconds have passed, rather, it just happily contines to point you in the wrong direction. Also, garmins don't have the capacity to reject satellite signals with low signal to noise ratios (SNR), high dilution of precision (DOP) or high multipath. What that means that regardless of the quality of a satelite signal or how many things it reflects off of before reaching your unit, the garmin will try to use it. Sometimes that means that although it's telling you that you have a position, the actual quality of that position can be highly questionable. Also, the way that it predicts you position's accuracy is somewhat questionable.

 

That being said, the Garmins have their place. For example, if you are above treeline and not on a north facing aspect, you position accuracy can be as little as 10 feet (and that's real!). That's why I think they are very useful for mountaineering. With enough satellites you can even get really accurate elevation readings! But, once you get below treeline, be careful using them to navigate. You may find that your navigation arrow jumps around as satellite signals are gained and lost. Anyway, my advice is to get a garmin (they are the cheapest and most user friendly), but just take it out and mess around with it under a variety of conditions. Also, if possible, try not to rely just on it as a navigation tool because the map and compass really works good, especially if visibility is good.

[gslater]

I'm a Garmin GPSMAP60C user (Magellan 315 before that), and guess I just haven't used it yet in a situation where I would have seen the 30 second delay in letting you know it can't get a fix. That might be because I've never used it in the wilds to continuously navigate to a waypoint. If I ever do need/want to use the GPS, I'm much more of a "where am I, and what's the bearing to the next waypoint?" kind of guy, and then I use a compass to get there. Of course, that behavior could also be different from one Garmin model to another, or one generation to another, since it's more of a matter of how the user interface is designed, rather than how the receiver itself works. I'll have to test it out sometime.

 

I guess I don't understand your comment about needing to be not be on a north-facing aspect to get the great positional accuracy. In general, it seems that it shouldn't be dependent upon that. In a former life, I worked in the world of, uhhhhhhh, let's just say "spacecraft control", including certain aspects of the GPS constellation. Can't remember the exact particulars of the orbital mechanics of the constellation, but from what I recall, it doesn't seem like it would make much/any difference whether north- or south-facing. I bet 30 seconds with Google would get me some pretty reasonable data though....

[sweatinoutliquor]

I guess this qualifies as a bit of thread drift:

 

I have heard that the GPSmap garmins work a bit better, although I have never used one myself. To answer you question (tell me if I'm wrong cause it sounds like you are pretty educated on the subject! ) regarding aspect: Satellite trajectories don't tend to pass over the north pole, which when you examine a skyplot of their trajectories, excludes a big patch of sky, generally facing to the north of your position. Here is a skyplot showing satellite trajectories from a recent planning alminac I downloaded:

 

 

Notice that just north of the center position there is a big gap in satellite coverage. If you are on a north facing slope, even if it's only 30 degrees, the mountain or hill masks most of the skyline to the south, and somewhat to the east and west. That means that in general, you will have fewer satellites available. Again, please let me know if that doesn't seem right cause I'm just going off some stuff I've read. Sorry to the original poster for thread drift... Hopefully your original question has at least been partially addressed!

[gslater]

I think that plot is a little bit deceptive, because it only represents where things can be seen in the sky from a given point. Looks like a northern hemisphere point. But it does show the satellite plot not going to the N Pole.

 

A little digging on the intarweb found 6 symmetric orbits, each with an inclination of 55 degrees, at something like 26,000 km from Earth center (sort of a medium orbit, by standards). Mean Earth radius is a bit more than 6000 km. Given that geometry, I think that puts them pretty far out there, and thus solidly visible over the poles, but obviously not right over the poles. Would need to plot that one out to be able to visualize it better.

 

But with the 55 degree inclination, at any given time satellite visibility/availability will probably indeed be very slightly better at non-polar latitudes, but I think it would be a pretty small effect. Maybe it's better to state it as a potentially lower availability at higher latitudes, rather than a northern-aspect thing. But yeah, it seems you're right that there could be some north-side degradation (in the northern hemisphere, of course), albeit very small.

Edited December 16, 2006 by gslater

[gslater]

Found a web site that has a couple of nice graphics of the constellation geometry and ground track.

 

GPS geometry

[daler]

GPS units work great in conjuction with map a compass- One of the big reasons people don't use them much is they don't know how to pull/pur waypoints off/on the map in the field. With out this skill the units are only only good for following the waypoints you add while walking or have put on at home on the computer.

 

If you work in UTM and have a universal utm grid reader you can do tons with them in the field-

 

Cheers,

 

Dale

[Bigtree]

I've had a couple of different GPS units over the years for work an play. Am currently using a Garmin GPSmap 60csx with MapSource topographic mapping software. The mapping feature on the device is excellent and has proven its worth to me on several occasions. Its also got expandable memory (a key feature in my mind) which has allow me to load/pack topos for pretty well all of Western North America, some down to 24K. Its got a compass and barometer that I never use (I still prefer to rely on my Silva and Thommen). Picked it up from http://www.gpscentral.ca/ (they are the cheapest store by far). I looked at the Rino line from Garmin but already have a variety of hand-held radios so no need go there.

[sweatinoutliquor]

Very cool website gslater! The alminac I downloaded there was for Corvallis, so right around 45 North. I do a fair amount of work in the forest with a Trimble unit, and although I generally can observe a N face apect affect, it's likely confounded with the high multipath environment of closed canopy forests. I still think that there may be an affect on satellite geometry because the available satellites may only occur higher on the horizon, but as you say, this may be hard to quantify and possibly insignificant.

[btowle]

In general, there are more satellites south of us than to the north. We do land surveying. If we need to obtain coordinates on a point in a field near the edge of a tree line, if the trees are north, it will probably work, but if the trees are to the south it won't. The geometry of the satellites will not be good in either case for survey grade accuracy, but in general is true.

[gslater]

In general, there are more satellites south of us than to the north. We do land surveying. If we need to obtain coordinates on a point in a field near the edge of a tree line, if the trees are north, it will probably work, but if the trees are to the south it won't. The geometry of the satellites will not be good in either case for survey grade accuracy, but in general is true.

 

Either of you guys have any experience that tells you which of the following is more important for ultimate accuracy, such as required for surveying work?

 

1) Having a greater number of visible satellites all clustered together toward one half of the sky, such as near the southern horizon?

 

or

 

2) Having good "triangulation", with satellites visible in multiple directions, even though there may be fewer total visible at the moment?

[sweatinoutliquor]

#2 seems to work better... It's all about having good geometry... With the Trimble I use, I can have 6 satellites locked, but an unacceptable level of precision. Usually that happens when a bunch of them are really close together, or they occur in a straight line across the skyplot.

[Crevasse]

I used to think old school and felt my compass was most important and gave my partner a hard time for fooling around with his gps. But now after a few, less than 50’ whiteouts I would not go out without it. I program in waypoints before I head out, log tracks for retreat mark bivy’s for future reference, and check general metrics along the way to distract from long slogs. I think they are great. I also use a rhino for the radio feature.

[dbconlin]

I work with GPS/GIS professionally quite a bit and have not felt that they would help me in the backcountry significantly enough to justify the 1) expense and 2) weight/bulk.

 

That is in part due to my confidence in map/compass skills and in part due to the hassle of getting good satellite geometry and accuracy with GPSes. It also absorbs more time in the field (that could otherwise be spent making progress on the route) than I am willing to give a tool that improves my navigation only negligibly.

 

That said, I would recommend newcomers to be very comfortable with map and compass. Then, if you want to add GPS capability, make sure you know how to use the damn thing. I have been on trips with people who tended to rely too heavily on GPS and obviously did not really know what they were doing. That is not the right approach.

[Ovr40]

Obviously a compass is more reliable than a GPS, but a compass can fail (I had one lose liquid and become unuseable) or be lost. The compass function on a GPS provides a backup in this case.

I would recommend newcomers to be very comfortable with map and compass.

Absolutely. And this requires practice.

[jfmctlaw]

What good is a map/compass in a white out? Yes, it's nice to know your heading but it's pretty hard to know where you are without some landmarks or sightings....agree?

[Gary_Yngve]

With an altimeter, you also have elevation. You can identify local slope curvatures to figure out where you are on a map. And often for navigation you don't need to pinpoint your location, as long as you know that there's a route from your current location to the next "wayarea" ahead.

 

Coming down from Camp Muir in a whiteout is a classic place where a compass is useful.

[gslater]

Coming down from Camp Muir in a whiteout is a classic place where a compass is useful.

 

As is coming down the south side of Hood. 180 mag, if I recall correctly...

 

 

 

jfmctlaw - In a whiteout you can't exactly use a map and compass to figure out where you are, but you can certainly use the compass to ensure you're headed in a certain direction. You could also use the GPS to figure out where you are and which direction to head to the next target, and use the compass for actual en-route navigation.